Condensation on windows and doors

The amount of internal or external condensation on parts of buildings has a clear relationship with the thermal insulation performance of the material. The three types of condensation have different underlying causes:

  • Internal condensation is normally caused by poor thermal insulation and high internal relative humidity. Its occurrence increases at high values of indoor relative humidity, low outdoor temperatures or poorly insulating windows, and decreases with improved insulation performance of windows.
  • Condensation inside a window is caused by a weakness in the design of the window or by an unsuitable pressure difference across the window, allowing warm moist air to leak into the inter-pane space and condense on cold surfaces.
  • External condensation can occur on well-insulated windows during cold clear nights in combination with high relative humidity of the outdoor air. The amount of condensation depends on a combination of ambient conditions, the building structure and climate. The most important factors are screening of the window from the sky, a high outdoor relative humidity, low heat flow through the window, and a low U-value of the window.

See the article, 'Calculating the occurrence of external condensation on high-performance windows', for a more detailed explanation of the effects of the various parameters.


SP Report 1995:01: External condensation on windows:  Measurements in Borås, 1994 (in swedish)

External condensation on windows. Bygg & Teknik no. 8, 2000, pp. 46-49 (in swedish)

External condensation. Presentation at the Focus on Windows conference, 13th November 2002 (in swedish)

SP AR 1999:40: Calculating the occurrence of external condensation on high-performance windows.

Related Information

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